Majma al-Zawaid — Hadith 15035

Al-Bahi narrates: I said to Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with them both): "Tell me about the person who most closely resembled the Prophet (peace and blessings be upon him)." He replied: "Sayyiduna Imam Hasan bin Ali (may Allah be pleased with them both). He most closely resembled the Prophet (peace and blessings be upon him) and was the most beloved of people to the Prophet (peace and blessings be upon him). He would come while the Prophet (peace and blessings be upon him) was in prostration and climb onto the back of the Prophet (peace and blessings be upon him), and the Prophet (peace and blessings be upon him) would not rise until he moved aside. Once he came and went in under the belly of the Prophet (peace and blessings be upon him) (that is, between his legs), and the Prophet (peace and blessings be upon him) spread his legs apart for him, so he came out from the other side."
Al-Bazzar recorded it; its chain contains Ali bin Abis, who is da'if (weak).